Magnetic sensing devices have many applications, including navigation, position sensing, current sensing, vehicle detection, and rotational displacement. There are many types of magnetic sensors, but essentially they all provide at least one output signal that represents the magnetic field sensed by the device. The Earth, magnets, and electrical currents can all generate magnetic fields. The sensor may be able to detect the presence, the strength, and/or the direction of the magnetic field.
The strength of the magnetic field may be represented by a magnitude and a polarity (i.e., positive or negative). The direction of the magnetic field may be described by its angular position with respect to the sensor. Magnetic sensors measure magnetic fields to determine position related parameters such as shaft rotation, presence of magnetic ink, vehicle heading, etc. One of the benefits of using magnetic sensors is that the output of the sensor is generated without the use of contacts. This is a benefit because over time contacts can degrade and cause system failures.
One type of magnetic sensor utilized in many conventional sensing applications is a magnetoresistive (MR) sensor. MR sensors are a type of magnetic sensor that uses the magnetoresistive effect to detect a magnetic field. Ferromagnetic metals, such as the nickel-iron alloy commonly known as Permalloy, alter their resistivity in the presence of a magnetic field. When a current is passed through a thin ferromagnetic film in the presence of a magnetic field, the voltage will change. This change in voltage represents the strength or direction of the magnetic field. By designing an MR sensor in a Wheatstone bridge configuration, for example, either the strength or direction of the magnetic field can be measured. The characteristic high-sensitivity and accuracy make MR sensors well suited for precision applications.
Magnetoresistive sensors are utilized in many automotive and aerospace applications. In automotive applications, for example, magnetoresistive sensors are often utilized for sensing cam and crank shaft targets. A magnetoresistive sensor system that may be utilized for sensing shaft rotation or linear displacement is depicted in FIG. 1. Specifically, FIG. 1 illustrates an anisotropic magnetoresistive (AMR) sensor system generally comprising an MR bridge 2, differential signal amplification and adjustment circuit 4 and an output signal detect module 6. MR bridge 2, comprising multiple AMR elements configured in a Wheatstone Bridge arrangement, delivers the differential signal, typically about 30 mV peak to peak, sensed across the bridge to amplification and adjustment circuit 4 where the sensed signal is amplified and centered at a specified reference voltage point using standard operational amplifier techniques.
The resultant signal, Vout, is processed by detector module 6 which employs signal conditioning circuitry to provide an accurate determination of the target position of the linear or rotational position and displacement. The primary function of the signal conditioning circuitry within detector module 6 is to capture the peak and min values of the amplified signal, and use these values to generate a threshold voltage signal that is the mid point of the captured peak/min values.
FIG. 1B is a waveform diagram illustrating exemplary signals employed by detector module 6 for conditioning the sensed output signal, Vout. As shown in FIG. 1B, sensor output signal 22 is a single ended signal and varies between peak and min amplitudes and represents the amplified AMR bridge signal. Detector module 6 translates sensor output signal 22 into a corresponding detected output signal 28 with the use of a threshold signal 25 by outputting a signal high when the value of sensor output 22 is above threshold 25 and outputting a signal low when the value of sensor output 22 is below threshold 25.
Threshold signal 25 is determined by averaging a max or peak signal 24 with a min signal 26. Peak signal 24 is determined in accordance with the sensed peaks and min signal 26 in accordance with the min values of sensor output 22. In this manner, threshold signal 25 is able to follow sensor output 22 which is very important in AMR sensing applications in which the MR sensor output is subject to drift due to operating and environmental factors such as temperature changes. It will be noted that during each cycle the peak signal 24 is “pushed down” and subsequently reacquires at the beginning of the next peak of sensor output 22, and conversely, min signal 26 is “pushed up” and reacquires at the beginning of the next min value of sensor output 22. This periodic re-adjustment is necessary to adaptively track the peaks and minimums of the sensor output.
A device that may be employed by detector module 6 for detecting and adjustably tracking the peak signal 24 is depicted in FIG. 1C. Specifically, FIG. 1C illustrates a signal tracking device generally including a comparator 18 which compares the amplified sensor output, Vout, with the presently stored peak value. A digital storage device in the form of a counter 14 is utilized to store the peak value with the output from comparator 18 used for incrementing the presently stored counter value when the signal from AMR sensor 12 exceeds the value of the presently stored peak value. Counter 14 is periodically updated by the output from comparator 18. The stored reference value is applied in the depicted example as a 9-bit output to a 9-bit digital-to-analog converter (DAC) 16 which converts the digitally stored value to the corresponding output peak boundary value which is applied to the input of comparator 18 for comparison with the real time sensor output on the next counter clock cycle. Although not depicted, it will be readily understood that a min detector can be similarly constructed by the use of a decrementing counter and switching the respective comparator inputs.
The mixed signal peak detector device depicted in FIG. 1C addresses analog leakage problems since the peak reference value is digitally stored in the period between sensor output peaks. However, the design of this peak detector is problematic in terms of its practicable application as a high-resolution, high slew rate tracking and low power device. Since the output from DAC 16 varies discretely in conformance with digital increments, the peak reference signal follows discrete steps determined in accordance with the DAC design. In the depicted example of a 9-bit DAC 16 and assuming a DAC range of 2V, the minimum resolution of the peak signal is 3.9 mV. For a 7-bit DAC having the same range, the resolution is 15.6 mV.
A resolution having fine granularity is generally desired to track the peak with maximum real time accuracy but is limited by the clock speed in terms of its ability to track a high slew signal. For example, if the clock speed is 10 MHz and a 9-bit DAC having 3.9 mV steps is used, the fastest slew rate that can be tracked is 39,000V/second while a 7-bit DAC can track at 156,000V/second. This tracking speed versus resolution tradeoff cannot be addressed by increasing the clock speed since, in practical application, a fast settling DAC requires a lower RC time constant and is incompatible with the low power requirements (typically less that 5 mA) of the application specific integrated circuits on which the detector circuits reside.
From the foregoing, it can be appreciated that a need exists for a low power sensor output amplitude threshold tracking device that provides a high-resolution capability with high slew rate tracking. The present invention addresses such a need.